For an operator to efficiently control a piece of machinery, the seat which provides the operator an interface with the machine must be able to comfortably support the operator over an extended period of time. Over the years a number of different seat suspension designs have been developed to provide a comfortable support for an operator. In the majority of those designs, the seat suspensions are constructed to have the amount of resistance or support provided, and the vertical position of the seat adjusted to accommodate the weight of the operator through the use of various suspension mechanisms or systems positioned within the seat. These mechanisms typically use springs, dampers, linkages and other devices to adjust these characteristics of the seat to comfortably position the seat with respect to a particular operator.
However, most seats constructed today are not capable of adjusting the amount of support provided to a wide range of operators in a manner that does not sacrifice the comfort, more specifically the vibration attenuation of the seat to a particular population of operators. The spring rate generally must be low to accommodate the light operator mass and the damping rate must be high enough to limit end stop impacts of larger mass operators. Since these attributes are generally not adjustable in low cost seat suspensions, vibration isolation is compromised over the mass adjustment range. Seat vibration test standards such as ISO7096 can be difficult to pass with a nonadjustable spring rate and damping rate suspension. To accommodate a light mass operator the spring rate and damping are generally low. The low damping rate in turn results in suspension end stop impacts causing the designer to either increase the damping at the expense of the light operator's vibration isolation or increase the suspension stroke at the expense of SIP (seat index point) height.
As a result, it was advantageous to develop suspensions for a vehicle seat in which the operation of the suspension is adjustable in order to achieve desirable ride comfort for the seat for both a low weight occupant and a high weight occupant. To this end, a number of adjustable suspensions have been developed, with a pair of notable examples being disclosed in Meiller et al. U.S. Pat. Nos. 5,261,724 and 5,490,657, which are incorporated by reference herein in its entirety. In these patents and other similarly designed adjustable suspensions, the preload on a spring utilized in the suspension can be adjusted such that the spring, and thus the suspension, will provide more or less resistance to movement of the seat depending on the weight of the occupant. More specifically, these types of seat suspensions include a pivot plate that is connected to the spring and is fixed about a pivot point on the seat. The adjustment made to the spring preload in the suspension is made by changing in the distance over which the spring acts from the pivot. Thus, the adjustment involves moving the pivot plate to reposition the spring further from the pivot to increase the suspension preload, or closer to the pivot to decrease the preload.
Another patent of note is Wahls U.S. Pat. No. 6,186,467, which is also incorporated by reference in its entirety. In this patent the preload of a seat suspension is changed by adjusting the position of a slide to which one end of each of a pair of springs is connected. The movement of the slide increases or decreases the spring length such that the preload for the suspension can be adjusted as required for a particular occupant.
While these suspensions can adjust the support provided by a seat to an either high or low weight occupant, this structure of the suspension has a number of shortcomings. First of all, this suspension does not vary both the damping rate and the spring rate of the suspension in proportion to the suspended mass in order to provide an equally effective resistance from the suspension to any weight occupant sitting in the chair. As a result, these improved prior art suspensions have a high weight seat occupant transmissibility curve that is significantly different than the transmissibility curve for a lower weight seat occupant. This is an undesirable result because the difference between the transmissibility curves implies that more effort is required to adjust the suspension than necessary, since during weight adjustment the heavy operator is gaining an advantage in isolation over the light operator. Assuming it takes energy to improve the vibration response of the suspension, and since the suspension should be able to pass tests in all weight adjust settings, the additional effort is wasted and should be minimized.
In order to have transmissibility and ride dynamics for a seat suspension that are equivalent regardless of the mass of the occupant, the damping rate, the spring rate, and the spring preload of the seat suspension would all need to be adjustable in relation to one another. Thus, it is desirable to develop a seat suspension that can provide the same effective ride to support to any occupant of the seat which can be accomplished by varying the damping rate, spring rate and suspension preload proportionally with regard to the suspended or apparent weight of the occupant. This is because by changing the spring rate and damping rate together in proportion to the apparent operator weight change, the transmissibility curves for the suspension for each apparent operator weight are kept coincident. The coordination of the spring and damping rates should also allow the work required to change the weight setting for the suspension to be minimized.
Another shortcoming of a number of these prior art adjustable seat suspensions is that, in seat configurations where the suspension is disposed beneath the seat, the overall seat height and seat index point (SIP), is too high for many applications. The SIP is measured using a standard measurement device well known in the art that essentially represents the location of the hip pivot of the seated occupant. This is a point in space that serves as a reference point for the positions of other structures in the vehicle, such as the pedals, the steering control, etc., that can be used in order to apply ergonomic principles to design the interior of a vehicle. If the SIP location positions an operator too close to the controls of a vehicle, an occupant will not be able to get into the seat without striking the controls, possibly damaging the controls, injuring the occupant and/or causing the vehicle to inadvertently move in an uncontrolled manner.
Also, the prior art suspensions used a spring with a low spring rate which requires significant deflection of the spring to adjust the preload of the suspension an adequate amount for a wide range of occupant weights. This long deflection of the spring requires significant space under the seat, consequently increasing the size of the suspension and of the seat incorporating the suspension.
An additional shortcoming of many seat suspensions that adjust the suspension preload to accommodate operator mass by directly increasing the spring preload is the effort required to make said adjustment. The adjustment often requires multiple turns of a hand-controlled knob which requires increased torque to be applied as the spring preload increases. Therefore, it is desirable to develop an operator mass adjustment mechanism that requires the operator to impart a sufficient low and constant force through the adjustment range.
Further, in those seat configurations where the suspension system is positioned within the backrest of the seat, such as in the seats disclosed in the Meiller et al. patents, the problems with an SIP that is too high are obviously not present. However, these configurations have other drawbacks in that the backrest is severely limited in its ability to be reclined or folded forwardly over the seat which is useful for vehicles in which a fuel storage tank is disposed behind the seat, such as a lift truck. In addition, the depth of the backrest can limit the available fore/aft adjustment range of the seat suspension assembly.
Therefore, it is also desirable to develop a seat suspension that can be disposed either under a seat or in a backrest as required to provide the desired type and range of motion for the seat on the particular vehicle. Also, the suspension should require a sufficiently low deflection length for the springs in the suspension to significantly decrease the packaging envelope of the suspension and reduce the effort required to adjust the suspension preload. This reduction will allow for under-seat placement of the suspension such that the seat is positioned at a sufficiently low SIP while also allowing the backrest to recline fully, move forwardly, and rearwardly, and have the feature of folding over the seat.